1. Field of the Invention
The present invention mainly relates to an optical transmitter and receiver module used for optical communication.
2. Description of the Background Art
In terminal equipment of an optical access line, referred to as FTTH (Fiber To The Home), used in households, there are used optical transmitter and receiver modules which conduct bi-directional transmission with a single fiber. As an example of a conventional optical transmitter and receiver module, for example in FIG. 1 in Japanese Unexamined Patent Publications No. 7-261054, there is shown an example (a first conventional example) in which a module accommodating a laser diode and a photodiode in the same package is used.
In the example shown in the above Japanese Unexamined Patent Publications No. 7-261054, a lens is located between an aperture of the package in which a laser diode and a photodiode are accommodated and an optical fiber and a cover glass of the aperture of the package is provided with a diffraction grating. In the optical transmitter and receiver module thus constructed, disclosed in the above Japanese Unexamined Patent Publications No. 7-261054, light has outgone from a laser diode passes through the diffraction grating and is condensed into the optical fiber by the lens. On the other hand, in reception, light has outgone from the optical fiber reaches the diffraction grating through the lens. This light is then diffracted at the diffraction grating and condensed at a photodetecting surface of the photodiode.
And, in Japanese Unexamined Patent Publications No. 8-15582, there is disclosed a conventional example (a second conventional example), differing from the first conventional example, in which a lens is located in the immediate vicinity of a laser diode to make light from the laser diode a parallel rays of light and a diffraction grating having a lens function allows this parallel rays to enter an optical fiber.
However, in the first conventional example, there is a limit to that the laser diode and the photodiode are located close to each other when considering the sizes of these devices themselves, and it is difficult to cause them to approach each other within a distance of about 0.2 mm. For the purpose of allowing a laser light to enter an optical fiber and allowing light outgoing from the optical fiber to enter a photodiode by retaining a required space between the laser diode and the photodiode, there are a method of increasing a diffraction angle of a diffraction grating (a first method) and a method of lengthening a distance between a diffraction grating and a laser diode (a second method). However, in the first method, the diffraction grating, in which the diffraction angle is increased, decreases in a diffraction efficiency and causes a problem that a coupling efficiency of a laser light for an optical fiber is deteriorated or a photo-electrical conversion efficiency of light outgoing from an optical fiber in a photodiode is deteriorated. And, when a distance between a lens and a laser diode is lengthened like the second method, aberration becomes large and a problem that a coupling efficiency of a laser light for an optical fiber is deteriorated arises.
Further, in the second conventional example, there is a problem that since a diffraction grating has a lens function, it becomes a diffraction grating in the form of a circle band having a short period and therefore a diffraction efficiency is deteriorated.